Project Summary/Abstract Vitamin A is critical for human health and vision. Insufficient uptake of vitamin A severely damages photoreceptors, causes a loss of visual pigments, and is the leading cause of preventable childhood blindness according to the WHO. However, little is known about how vitamin A deprivation affects photoreceptors on the molecular level and how vitamin A replacement therapy mediates structural and functional photoreceptor recovery. Our long-term goal is to use the Drosophila melanogaster retina as a model to fill this gap in our knowledge and to analyze the underlying mechanisms. This proposal synergistically combines Drosophila genetics, immunohistochemistry, behavioral analysis, and quantitative proteomics. This multidisciplinary approach will allow us to evaluate the central hypotheses that different photoreceptor types ? functionally equivalent to human rods and cones - respond differently to vitamin A deprivation and that vitamin A deficiency triggers protective mechanisms that stabilize damaged photoreceptors. We will pursue three major specific aims. First, we will analyze how different photoreceptor types respond to vitamin A deprivation. Second, we seek to identify the proteins and cellular pathways that are affected by vitamin A deprivation and vitamin A replacement therapy. Third, we will determine the function of a novel transmembrane protein that we found to be highly upregulated in vitamin A-deprived retinas to stabilize the damaged photoreceptors. The proposed research is significant, as it will provide fundamental insights into the molecular response of different photoreceptor types to vitamin A deprivation. It will also unravel how vitamin A replacement therapy leads to improvement of photoreceptor structure and function. Collectively, this proposal has the potential to identify mechanisms that stabilize damaged photoreceptors and to open new avenues for treating human eye diseases.